Many different types of level sensors are available, and these rely upon a variety of different physical principles. One reason for the large variety of sensors is that level sensors are required in numerous different environments for many different purposes, and few level sensors are versatile enough to have a wide range of use. Thus, for example, some level sensors merely detect whether or not liquid has exceeded a predetermined level, whereas some applications require measurement of liquid level throughout a range. Some level sensors operate over an inadequate range, and others with inadequate resolution. Sensors which operate accurately over a predetermined range can be excessively expensive, particularly for wider ranges. In some cases, the structure or physical principles of operation are incompatible with the environment in which the sensor is intended to be used.
Some known forms of liquid level sensor are level switches which operate using optical principles. For example, the sensor may be a transparent device which incorporates a light emitter and a light receiver so arranged that light from the emitter is totally internally reflected by the outer wall of the device to the receiver when the sensor is surrounded by air. However, when immersed in liquid, the light from the receiver is refracted into the liquid, so the light received by the receiver diminishes, thus providing an output indicative of the immersion. See for example GB 2 036 326 A.
Such arrangements are not, however, suitable for indicating varying liquid levels. It would of course be possible to provide a number of such level switches spaced in a vertical direction in order to provide for a varying level indication. However, this would be expensive and difficult to assemble. The physical size of each switch would preclude the possibility of mounting them in close proximity, thus resulting in an arrangement which is lacking in resolution and/or compactness.
It would be desirable at least to mitigate the problems referred to above.